The planets will align for astronomy enthusiasts on Monday, May 9, as Mercury makes a rare visible transit across the sun.
Both professional astronomers and hobbyists can view the transit between 7:12 a.m. and 2:42 p.m. Eastern time on Monday, provided they have a telescope or binoculars fitted with a solar filter to prevent eye injury from the sun’s brightness.
“Astronomers get excited when any two things come close to each other in the heavens,” said Louis Mayo of NASA’s Goddard Space Center in a press release. “This is a big deal for us.”
Of the several planets in the solar system, Earth dwellers can only observe the transits of Mercury and Venus, since they are the only two planets between the Earth and the sun.
Some parts of the world won’t be so lucky on Monday, as skywatchers in Eastern Asia, Oceania, and Antarctica will be unable to see the transit, Astronomy Now reports. Viewers in the Western United States and Eastern Africa will only be able to see part of the event.
The first Mercury transit was observed in 1631 by astronomers with low-powered telescopes. Today, astronomers say that it is much easier to observe and record solar transits.
“It used to be hard to observe transits,” said Solar and Heliospheric Observatory (SOHO) project scientist Joseph Gurman in the release. “If you were in a place that had bad weather, for example, you missed your chance and had to wait for the next one. These instruments help us make our observations, despite any earthly obstacles.”
Solar transits are unusual. Although Mercury orbits the sun every 88 days, the planets rarely align so perfectly that viewers on Earth can observe the spectacle.
Visible transits typically take place in May and November, and on average occur around thirteen times per century. The last time this happened was in 2006, and the next two are slated to occur in 2019 and 2032.
Mercury’s cross-solar passage is not just intriguing for viewers, however, but also for scientists who rely on the data gathered during solar transits.
Three NASA telescopes will be tracking the transit in order to gather data on Monday. Two instruments at SOHO, the Extreme ultraviolet Imaging Telescope and the Michelson Doppler Imager, will be put in use for the first time in five years to take measurements.
Scientists say that these measurements can help them better understand not only Mercury, but the sun. In particular, the data gathered on Monday will help scientists determine the solar rotation axis and study changes in the sun over time.
Mercury’s transit will also help scientists calibrate and study the impact of the sun’s scattered rays on their astronomical instruments.
“It’s like getting a cataract – you see stars or halos around bright lights as though you are looking through a misty windshield,” said Solar Dynamics Observatory project scientist Dean Pesnell in a NASA press release. “We have the same problem with our instruments.”
For viewers without solar-filtered telescopes or enthusiasts who aren't in a place where they can see the transit, NASA plans to post a feed of images from the transit as it occurs at http://www.nasa.gov/topics/solarsystem/features/eclipse/index.html.